DE69617327T2 - Tires with circumferential cords for anchoring the carcass, process for producing such cords - Google Patents

Description

The present invention relates to pneumatic tires. More particularly, it relates to the construction of the beads thereof.

The role of the tire beads is well known: they are to ensure the "attachment" of the tire to the rim on which it is mounted. To this end, the cords or cables of the casing all converge in the lower part of the bead, where they are firmly anchored so that the casing can withstand the tensile forces during use.

Very recently, a new type of bead was proposed in patent application EF 0582196, which corresponds to the preamble of claim 1. This bead does not have the usual wrapping of the carcass around a bead core. Instead, at the anchoring point, the carcass reinforcing elements are arranged in one or more orientations. If one imagines the arrangement of the group of these elements in the tire, then inside each orientation they form a partial truncated cone with an axis that coincides with the axis of rotation of the tire. The reinforcing carcass elements are laterally adjoined by at least one stack of peripheral reinforcing elements, made for example by spiral spinning. In addition, a suitable binding rubber mixture ensures the transmission of forces between these reinforcing elements, which are aligned perpendicular to each other.

The applicant's tests have shown that such a bead structure fully withstands the stresses that occur during use, even in heavy use, and this is equally well suited to touring vehicles as to other applications. In addition to the stresses resulting from the actual use of the tyre, the tyre must be able to withstand an indefinite number of dismantling operations followed by re-assembly in order to follow up on use.

As is well known, the greater the clamping of the bead when the tire is mounted on a wheel, the less the tire is likely to come off the rim. Clamping is, to recall, the compressive force of the rubber located radially in the lower part of the bead, which exerts a pressure on the radially outer surface of the corresponding rim seat. A certain height of clamping is necessary to transmit a braking or driving torque between the rim and the tire. Clamping depends not only on the actual characteristics of the tire (bead geometry, rigidity of the materials used), but also on the geometry of the rim itself.

But it is also known that the greater the clamping, the greater the difficulty in fitting and/or removing the tyre. Dismounting in particular involves the application of a force that is quite significant (depending on the clamping on the rim) on the bead at the level of the rim lamp or immediately above it. This force is directed parallel to the axis of rotation and is always applied locally by a ram or a lever. These tools impose a deformation on the bead of the tyre. This deformation can be very significant. In the first step of dismounting, the aim is to break the grip of the bead, that is to make it leave its seat while removing it from the rim edge. During this first step, the bead of the tyre is subjected to localised but very high tensile forces.

Subsequently, levers are generally used to force the bead to come over the edge of the rim. In fact, in the case of one-piece rims, the shape of the rim (the most common case for touring and light truck tyres) is designed to allow mounting or dismounting by deforming the bead into an oval shape without increasing its circumference. This requires in particular the design of the central mounting recess and the edges which laterally delimit the rim and determine the mounting position of the bead. During this second step, the bead undergoes a total deformation which is considerably less damaging than the stresses during the first step.

The tire designer seeks to obtain a good compromise between safety (low sensitivity to detachment from the rim), achieved in particular by acting on the clamping, and ease of assembly/disassembly. The observance of these requirements (working loads and assembly/disassembly loads), which are somewhat contradictory, and the desire to simplify manufacture and limit the weight of the material are the objectives of the present invention.

More precisely, an aim of the invention is to improve the ability of the structure of the bead described in the aforementioned patent EP 0582196 to undergo disassembly even under less than careful conditions and in particular with misaligned tools. In this case, the tip of the tire, held by a hump, undergoes a rotation during the first step of disassembly that is essentially centered on said tip (see Fig. 3), because this type of bead is quite soft to rotation in a radial plane. When this rotation goes as far as a tilting movement of the tire, as shown in Fig. 3, part of the turns of the circumferential cable 2 undergo a very significant stretch. This stretch can be about 3% for the lowest turns 40 of the axially outer stack 4. In extreme cases, when this stretching is combined with a local deformation due to the pressure of the dismantling tool, dismantling can cause the rope to break in one or more turns.

The present invention particularly aims to allow a very large number of dismounting operations with a possible reuse of the tire, without sacrificing the performance of the tire in use.

According to the invention, the tire has sidewalls ending in beads, which in turn are designed to be mounted on a rim, and has a carcass reinforcement which runs through the sidewalls and converges in the beads, at least one of the beads having the following characteristics:

- reinforcing carcass elements running from the radially lower part of the bead to the sidewall,

- at least one stack of cords running in the circumferential direction and adjacent to the reinforcing carcass elements, said circumferential cords having a functional elongation rate Af = Ae + Ap greater than 4%, and

- a connecting rubber compound placed between the circumferential cords and the carcass reinforcing elements.

Fig. 4 shows a diagram of the stress as a function of the elongation. Firstly, one notices an elongation As, which is specific to the rope effect. This elongation expresses a yielding of the wires of the rope, even before the said wires are subjected to tensile stress. The diagram then shows a plastic elongation Ap and finally an elastic elongation Ae.

For the description of the invention, we introduce here the expression of the degree of functional elongation or the functional elongation rate Af = Ae + Ap. This functional elongation rate does not include the specific elongation As of the "rope effect".

As for the maximum voltage, it is expressed by the following formula:

where Fm is the maximum force, p is the mass per volume of the material considered (7.8 g/cm³ for the steel used) and M/L is the mass per length of the rope used.

Preferably, the maximum stress Rm is higher than 2000 MPa and even advantageously higher than 2200 MPa, which makes it possible to construct a bead that is as light as possible, lightness being in itself a factor of performance and helping to limit the cost of the tyre.

The invention will be fully understood by reference to the following description which, with the aid of the accompanying figures and in a non-limiting manner, represents a concrete embodiment of a tire bead.

Fig. 1 and 2 are radial sections of two variants of a tire bead according to the invention.

Fig. 3 shows the rotation of the bead during demounting of a tire.

Fig. 4 shows the different strains that characterize a rope.

Fig. 5 shows the characteristic reinforcing elements used in the tire beads.

Fig. 6 and 7 show embodiments of the invention.

Fig. 8 shows another variant that is subjected to rotation during the demounting of a tire.

In Figs. 1 and 2, one can see a bead in which reinforcing elements 1 of the carcass and circumferential ropes 2 can be recognized.

In this paper, the term "wire" is reserved for a monofilament resulting from a spinning or drawing process or a rolling or calendering process or an equivalent process. A rope is an assembly of several fine wires. In the case of steel wires, these are wires with a diameter of approximately between 0.05 and 0.8 mm. This paper does not cover the different manufacturing processes of such an assembly, which are numerous and known to the person skilled in the art. The term "rope" refers without distinction to all types of assembly of wires (for example, strands, twines or actual ropes).

When we speak of a "reinforcing element", we are first of all referring, in a general way, to ropes as well as monofilaments, depending on the appearance of the element in question, provided it is fibrous, and regardless of its material. For example, a rayon or aramid rope falls under this general definition.

The reinforcing elements 1 of the carcass are made of metal or non-metal cords or monofilaments, without distinction. The anchoring of the reinforcing carcass elements 1 is ensured by circumferential cords 2 interposed by a connecting rubber composition 3. Preferably, said composition is the "MS" mixture, the recipe of which is given in point 6 of the first table. The circumferential cord 2 is arranged in several turns forming one or more spirals. The reader can find more details concerning this type of bead in the aforementioned patent application EP 0582196. These cords 2 are in turn formed by the arrangement of several wires 21, which are clearly visible in the enlargements drawn in Figs. 1 and 2 inside circles drawn to the right of the bead.

To get an idea, one could use an unstretched rope (2+7)0.28. This rope is made up of a core made up of two twisted wires. is surrounded by a layer of 7 wires, also twisted in opposite directions. The exact arrangement model, however, has no particular influence on the implementation of the invention. On the other hand, the applicant's experiments have led to the observation that it was not without difference to use any steel cable.

Fig. 5 gives a diagram of tension as a function of elongation for a conventional rope (curve C), for a rope according to the invention (curve I), and for a bead core wire of a conventional type (curve T). As a general illustration, one can cite patent US 5010938, which shows a winding scheme of conventional type used to form a bead core. This is formed by winding several contiguous turns of a bead core wire. A steel wire with a fairly large diameter, of the order of 0.8 mm or even more, is called a "bead core wire". This bead core wire is often a high elongation wire and has a maximum tension Rm of about 2000 MPa (megapascals - see curve T in Fig. 5). The functional strain, which here corresponds to the total strain At, is increased and is in the order of 6%.

Curve C represents the characteristics of a steel cable after vulcanization in a tire, usually used in crown plies. One can note a functional elongation Af, which is about 2%, as well as a slight elongation As, which is specific to the "rope effect", i.e. to the architecture of the cable used. Failure occurs at a high stress level Rm, which can reach 3000 MPa.

The invention aims to obtain a high resistance to successive disassemblies while maintaining the advantage of the bead structure described in the above-mentioned patent EP 082196. Furthermore, the invention intends to obtain this resistance to disassemblies while using materials with an increased maximum stress, which reduces the The amount of material used is limited, thus making the tire lighter. Another advantage of the invention is that the tire is easier to manufacture due to the use of cords, as the cords are less difficult to insert (because they are soft) than larger diameter wires such as the bead core wires.

The invention proposes the use of a very special rope 2 to form the stacks 4 of reinforcing elements. This rope enjoys at the same time a rather high maximum tension, at least as high as that enjoyed by the conventional bead cores, and a high elongation, much greater than that offered by conventional ropes.

It is now described how such a rope can be obtained by a special thermal treatment.

The influence of the thermal treatment on the change in the functional elongation Af is a function of the chemical composition and the degree of work hardening of the steel wire, the hardness and the temperature of the thermal treatment. In order to obtain a significant increase in the functional elongation Af, it is preferable that the degree of work hardening c of the wire used remains less than a value lying between 3.5 and 4, the exact limit depending on the chemical composition of the steel wire. The degree ε is defined by the following relation: ε = ln(S₀/Sf), where S₀ is the surface area of the wire cross-section before work hardening or cold rolling or stretching and Sf is the surface area of the wire cross-section after work hardening.

It is known that steel reinforcements for tires have a high tensile strength and a good adhesion to rubber. The resistance is obtained during the shaping of the steel wire by a process known to the person skilled in the art. process, for example by drawing. This operation, carried out on a thin wire, requires a drawing lubricant. In tire applications, this drawing lubricant is generally formed by a conventional adhesive coating, usually made of brass, applied to the steel wire to promote adhesion of the rubber to the steel wire. According to a variant, the adhesive coating could also be made of a Cu, Zn and Ni-based alloy, or any coating which promotes adhesion to the rubber while also playing the role of drawing lubricant. Several wires can then be joined together to form a rope. This gives a reinforcement made of work-hardened steel wire covered with an adhesive coating. These reinforcements are characterized by a low extensibility of the assembly and/or of the wires which make up the assembly (see curve C in Fig. 5).

A thermal treatment carried out after work hardening to increase ductility is known per se. The cladding process (for example a brass coating) is generally carried out after the thermal treatment in order to avoid deterioration of the cladding.

For example, patent FR 2152078 describes a means of improving ductility by modifying the structure. It describes how to obtain a material with a tempered martensitic structure. In this patent FR 2152078, a temperature level of the order of 800ºC is exceeded. The brass coating can only be applied after this type of thermal treatment, because otherwise the temperatures reached would destroy the brass coating and this reinforcement would be unusable in the tire because it would not adhere to the rubber. This patent therefore describes a thermal tempering treatment applied to a substantially martensitic structure and a quenching. In addition, because the tempering is carried out in a lead bath, a cleaning operation which will be very problematic if one chooses to apply this process to assemblies such as ropes and not to a monofilament (single wire), since cleaning is a very delicate operation when applied to twisted ropes. Finally, the traditional brass coating cannot be applied before tempering because the material is not sufficiently ductile. If it is applied to the rope after tempering, it is very difficult to ensure the homogeneity of the brass coating. It can therefore be seen that this patent does not provide a satisfactory solution for maintaining the adhesion of the rope to the rubber.

In the context of the present invention, various manufacturing processes are described which have proven to be particularly advantageous. These processes, which are of interest in themselves, are also preferably used for ropes whose steel wire has a carbon content of between 0.7% and 0.9%.

It is a thermal treatment by the Joule effect (hereinafter referred to as EJ) at a temperature of between 400ºC and 500ºC for a duration of 5 seconds or less. The times indicated are heating times; they do not include the cooling time. It is also possible to refer to the thermal treatment by static convection (hereinafter referred to as CV), where the convection is preferably carried out in a protective atmosphere and at a temperature of less than 420ºC, and in this case the subsequent cooling is also carried out in a protective atmosphere. It is also possible to refer to the thermal treatment by induction (hereinafter referred to as IN), where the temperature is between 400ºC and 550ºC and the heating time is less than or equal to 1 second.

Heat treatment by EJ or IN can also be carried out under protective atmosphere in order to limit as much as possible the deterioration of the coating (for example, the oxidation of brass). In this case, it is preferable to keep the rope in a protective atmosphere during cooling. As a variant or complement to the use of a protective atmosphere, these heat treatments can be followed by a pickling operation, which, as is well known, is followed by a water rinse and drying.

The present invention also extends to a heat treatment having in combination the following characteristics. This is a recovery annealing treatment carried out at a low temperature. This is understood to mean a temperature which must in any case be lower than Ac₁ (temperature corresponding to a transformation of the crystalline structure of the steel) and which is preferably less than or equal to 550°C, while it is generally higher than 250°C. This is also a treatment carried out directly on the cable comprising wires separately covered by an adhesive coating.

In reality, the temperature limit depends on the time and method of heating. It seems that the energy supplied to the rope must be essentially the same for all heat treatments. The temperatures indicated are temperatures reached on the surface of the rope. They can be measured, for example, by thermovision, or by a contact meter if this is possible. They are detected during the heat treatment itself, or immediately after it, if it becomes difficult to do otherwise in practice. This is the case here with the values indicated for the IN heat treatment.

The heat treatment increases the functional elongation capacity Af of the rope to a value higher than about 4%, while maintaining the resistance to tension at a level sufficient for the tire (maximum tensile stress Rm with a value of at least about 2000 MPa after heat treatment), as well as maintaining a sufficient ability to adhere to the rubber Curve I in Fig. 5 describes a typical characteristic of such a rope. It has been made clear that this is a supplement of the capacity to the functional elongation, since the values given do not show the elongation As specific to the "rope effect". Now the supplement of the functional elongation does not depend on the architecture of the rope for identical material, but depends essentially on the heat treatment.

The wire used is more generally of cold-hardened steel with a high carbon content (which is between 0.4% and 1.0% C) and optionally contains conventional elements such as manganese or silicon to promote certain specifically required properties, as is known to those skilled in the art, and also contains impurities in minor quantities. The shaping to the final diameter can be effected by any process, for example by drawing. The wires are assembled to form a rope by an appropriate assembly process (twisting or actual braiding or rope formation).

The treated rope is made of cold-hardened and coated elementary wires. The heat treatment on the rope (i.e. after assembly) allows all the wires to be treated simultaneously in a single operation.

The following examples describe the processes used and the results obtained. Wire made of essentially pearlitic steel, cold-hardened and brass-plated, forming an unbraided rope is used everywhere. Its exact composition, given in proportion to the steel, is as follows: carbon content 0.81%, manganese: 0.54%, silicon: 0.25%, phosphorus: 0.01%, sulphur: 0.01%, chromium: 0.11%, nickel: 0.03%, copper: 0.01%, aluminium: 0.005%, nitrogen: 0.0033%.

Example 1: Treatment by the Joule effect (EJ) for a rope (2+7)0.28:

The principle is to heat the rope as it passes by, constantly using the Joule effect, in a protective atmosphere (for example a mixture of nitrogen and hydrogen). The heating time is approximately 2.7 seconds. The treatment temperature is 450ºC. After heating, the rope is cooled in a protective atmosphere (N2, H2) and then wound onto reels. It has the following characteristics:

where in this example as in the following:

1. "TTBT" "low temperature heat treatment" means:

2. the adhesion values (adh) give an assessment of the quality of the bond between the rope and a rubber composition forming a block in which the said rope is embedded, the assembly being vulcanised, while a part of the rope is allowed to emerge from the said block so as to form a test sample; the values given correspond to the magnitude of the force necessary to extract the rope from the rubber;

3. all changes (A) are expressed in percentages; to allow the classification of the different solutions by a comparative reading;

4. the actual ability of the rope to adhere to rubber is monitored experimentally on the above-mentioned sample by observing the force at which the rope and the material separate;

5. the columns "test" correspond to a matrix or a starting material from a so-called test mixture, which has 100% NR (natural rubber), with the addition of reinforcing additives to obtain a suitable Shore A hardness, which is higher than 70, a significant proportion of sulphur, ranging between 5% and 8%, and an increased proportion of cobalt, ranging between 0.3% and 0.4% (the percentages indicated refer to the total weight of the elastomer); as for its ability to adhere to the rope, this mixture is very sensitive to chemical alterations of the brass coating on the rope;

6. the columns "MS" correspond to the preferred mixture described below with reference to patent EP 0582196. It is recalled that said mixture comprises a synthetic elastomer SBR (styrene-butadiene rubber) used alone or in blend with polybutadiene (PB), said SBR having a glass transition temperature (Tg) lying between -70°C and -30°C and said PB having a TG lying between -40°C and -10°C, said synthetic elastomer(s) being used in an aggregate proportion of at least 40% of the total elastomer weight and the balance being made up of natural rubber. The Tgs in question are measured by differential thermal analysis. Preferably, a mixture containing 50% of SBR solution with a Tg of -48ºC and 50% of NW is used, with the addition of reinforcing additives and resin to obtain a suitable Shore A hardness of over 70. Preferably, in order to obtain good bonding of the mixture to a brass-plated wire, a significant proportion of sulphur is used, ranging between 5% and 8% of the total weight of elastomers, and cobalt is used in an amount of the order of 0.2% of the total weight of elastomers.

Example 2: Convection treatment (CV) on a rope (2+7)0.28, followed by pickling (DECA).

The rope is treated in a static convection furnace (recovery furnace) under a controlled protective atmosphere, for example nitrogen/hydrogen with 6% hydrogen. It is heated up to 350ºC within 3 hours 30. It is then The result is that the same temperature is maintained for 30 minutes. Then it is cooled to room temperature in 3 hours. The reels are then unwound to allow the rope to pass through a bath of orthophosphoric acid or sulphuric acid at a very low concentration (about 4%). The residence time in the pickling bath is of the order of 2 seconds. The bath is at room temperature. The characteristics obtained are the following:

Example 3: Treatment by induction (IN) on a rope (2+7)0.28.

The rope is heated by induction as it passes through a protective atmosphere (cracked NH3 or N2, H2). The recovery annealing is carried out by electromagnetic induction by passing the induced currents over a length of around 40 cm, the treatment speed can be variable (for example 80 m/min) and the whole arrangement is regulated so as to obtain a homogeneous heat treatment of the rope. The temperature at the surface of the rope and at the outlet of the induction device is of the order of 450ºC. The properties obtained are the following: Example 4: Joule effect (EJ) treatment of a rope (3+8)0.35 under protective atmosphere.

The analysis of these examples shows that the absolute value of the adhesion of the rubber to the rope also depends on the formulation of the rubber compound used. It is thus possible to accept a greater or lesser variation in the adhesive coating due to the heat treatment, depending on the formulation of the compound used for the connecting rubber 3.

When using the above MS mixture, a greater deterioration (of the order of 70% for the "test" mixture) can be tolerated, since the adhesion performance achieved with the mixture is less sensitive to a change in the chemical nature of the coating during heat treatment at low temperature. However, preference is given to retaining only solutions that offer a deterioration of less than 50% for the test mixture. In all the above examples, and in particular in example 3, it can be noted that the MS mixture offers the best ability to keep deterioration at a low level in the most unfavourable cases. However, recipes could be found for other types of mixtures and this last remark is not limiting. Other conditions for carrying out heat treatments could be used; it is possible that in certain cases a lesser deterioration in adhesion could occur, which would make it possible to use the "test" mixture mentioned for layer 3, or even to use another mixture that is less favorable for adhesion than the MS mixture.

In summary, the invention proposes a process for manufacturing a cord for a tire, in which one starts from a steel cord hardened and covered with an adhesive coating which promotes adhesion between the steel and the rubber, and subjects said cord to a heat treatment of recovery annealing, at a temperature lying between 250°C and Ac₁, for a preselected duration and in such a way as to bring the degree of functional elongation Af to a value greater than 4%, using means such that, when one monitors the ability of such a cord to adhere to a so-called "test" rubber mixture before and after said manufacture, the deterioration observed is less than or equal to 70%.

The means mentioned may be pickling of the steel cable after the low temperature heat treatment (TTBT), or the choice of a sufficient protective atmosphere during the TTBT and the subsequent cooling, or any other means with equivalent effect.

Thanks to the invention, good adhesion between rubber and metal is maintained. In addition, in the event of significant stresses during disassembly, the cable exceeds the elastic limit, but always remains below the breaking point. The cable 2 therefore maintains a zone of elastic behavior even where it has borne a local force beyond its initial elastic limit. Even if a significant residual elongation (of the order of 2 to 3%) remains in the zone stressed by disassembly, this elongation, transmitted to the entire circumference of a cable turn 2, only offers a value of the order of 1 per 1000, which in no way affects the good durability of the tire on the rim after reassembly.

The utility value of the tire thus manufactured is not altered by several cycles of disassembly and assembly in any way other than that of a tire during its use, all the more so because, even if an elongation occurs which exceeds the elastic limit during each dismantling, not only does this elongation affect only a very short length of the rope, as has been seen, but, moreover, such elongation occurs only in a small proportion of the superimposed turns, and preferably in the axially outermost stack of the bead.

Above, it has been proposed to use a very special rope with high elongation as a circumferential reinforcement, which ensures the anchoring of the reinforcing carcass elements. Below, it is proposed to observe geometric construction rules for the architecture of the bead. These rules can be used in combination with the type of rope outlined above or just as well independently of this type of rope. These construction rules can therefore be used whatever the circumferential reinforcing element may be, even if rope 2 is replaced by textile ropes or monofilament reinforcements, whatever their nature and geometry. It should be noted that preferably the circumferential reinforcing element must have a sufficiently high elongation capacity. If a bead core wire (metal monofilament) is used, then this is a wire with high elongation (see, for example, curve T in Fig. 5).

If, for example, the dimensioning of the bead and/or the choice of materials make the bead relatively less compact, it is advantageous to offset the bottom of the axially outermost stack 5 radially upwards, as shown in Figures 6 and 7. Thus, if there are at least two stacks of reinforcing circumferential elements 2 laterally adjacent to the reinforcing carcass elements, there is a radial offset between the radially lowest part of said stack 5 and the adjacent stacks 4. This also helps to limit the stress on the cables during dismounting of the tire. Preferably, the radially lowest part of each stack is radially upwards with respect to the radially lowest part of the adjacent stack on the axially inner side.

In the same sense, the lowest turn of each stack 41, 42, 43 (see Fig. 8) is located at such a radial height that, during a tilting movement of the bead, it is not so stressed that its diameter does not increase by more than a value that limits the elongation of the cable to a height that it can absorb without damage. In other words, the overstress of the turns subjected to the greatest stress remains low. It should be noted that the increase in the circumference of the turns concerned is not proportional to what may appear as an increase in diameter in Fig. 8. In fact, it has been explained above that the deformation does not affect the entire circumference of the turn in question homogeneously. The stress caused by the tilting movement of the bead is a local stress. It should also be noted that at the moment when the bead rests on the tip (central position in Figs. 8 and 3), the tip deforms, particularly under the effect of the shear stresses which at that moment act on a relatively small thickness, which limits the expansion to which the various turns are subjected.

From another perspective, it is possible to calculate more precisely the geometry of the bead, even the different hardnesses that may be encountered. In figures 6 and 7, the total distances e₁ and e₂ have been shown, where e₁ is the distance in the radial direction separating the lowest turn from the surface of the bead before it comes into contact with the seat of the rim (or its extension, as the case may be) and e₂ is the distance in the axial direction separating the lowest turn from the surface of the bead on the side of the inner cavity of the tyre, i.e. where it meets the radially inner side. In order to avoid damaging overstress on the winding in question, it is advisable, as a first approximation (that is, if we assume that the hardness of the different products crossing each other along the segments e₁ and e₂ marked in Figs. 6 and 7) that e₂ should be less than or equal to e₁. If we take into account the exact nature of the various components more precisely, and consider that e₁ is the thickness of each product which radially covers the radially lowest winding of any stack of the radially lower surface of the bead, and that e2i is the thickness of each product that radially separates the radially lowest turn of any stack from the axially inner surface of the bead, and that G1j and G2i are the respective elastic moduli of the products considered, then the criterion of the concept can be expressed as follows:

e2i * G2i ≤ e1j * G1j

An architecture is proposed in which each turn of the stacks 4 or 5 tends to reduce its stress level during the tilting movement of the bead, which leads to a reduction in the level of clamping and to facilitate disassembly. The rules proposed are experimental. The result obtained is that, during tilting, the bead does not undergo any harmful stretching. Once the bead has left its seat, it relaxes, even on the axially inner side of the wheel, where the seat is often extended by a generally cylindrical, fairly large zone (the cavity of the rim is axially offset outwards). It is therefore essential that during the tilting movement each turn is not so stressed that it passes into a radial position higher than its reference position in the non-rotationally loaded bead (see Fig. 8).

Claims (24)

1. Tyres with sidewalls ending in beads which are in turn designed to be mounted on a rim, the tyre having a carcass reinforcement (1) which runs through the sidewalls and connects the beads to one another, at least one of the beads having the following characteristics: - reinforcing carcass elements extending from the radially lower part of the bead to the sidewall, and - a connecting rubber mixture (3) arranged between the circumferential cords and the carcass reinforcing elements, characterized in that - there is at least one stack (4) of ropes or cords (2) which run in the circumferential direction and adjoin the reinforcing carcass elements, said circumferential ropes having a functional elongation rate Af which is greater than 4%. 2. Tire according to claim 1, wherein the circumferential cable (2) is formed from uniform, brass-coated wires of tempered steel (21). 3. Tire according to one of claims 1 to 2, wherein the circumferential cable (2) is arranged in several turns forming one or more spirals. 4. A tyre according to any one of claims 1 to 3, wherein said compound rubber mixture (3) comprises a synthetic elastomer SBR (styrene-butadiene rubber), which is used alone or in blend with polybutadiene (PB), said SBR having a glass transition temperature (Tg) ranging between -70ºC and -30ºC and said PB having a Tg ranging between -40ºC and -10ºC, said synthetic elastomer(s) being used in a combined proportion of at least 40% of the total weight of the elastomer, the balance being natural rubber, and said blend having a level of sulphur ranging between 5% and 8% of the total weight of the elastomer and of cobalt in an amount of 0.2% of the total weight of the elastomer. 5. Tire according to one of claims 1 to 4, wherein said cords have a maximum tension Rm higher than 2000 MPa. 6. Process for manufacturing a cord for a tire, in which a cord is started from a cold-hardened steel wire, the carbon content of which is between 0.4% and 1.0% and which is covered with an adhesive coating that promotes adhesion between the steel and the rubber, said cord is subjected to a recovery annealing heat treatment at a temperature of between 250°C and Ac₁ for a duration preselected in such a way as to bring the degree of functional elongation Af to a value greater than 4%, and means are used to limit the deterioration of the ability of such a cord to adhere to a rubber mixture. 7. Process according to claim 6, using a steel whose carbon content is between 0.7% and 0.9%, wherein said heat treatment is carried out by the Joule effect and for a duration less than or equal to 5 seconds, at a temperature between 400°C and 500°C. 8. A process according to claim 6, using a steel whose carbon content is between 0.7% and 0.9%, wherein said heat treatment is carried out by static Convection is carried out under a protective atmosphere at a temperature below 420ºC and is followed by cooling under a protective atmosphere. 9. Process according to claim 6, using a steel whose carbon content is between 0.7% and 0.9%, wherein said heat treatment is carried out by induction at a temperature lying between 400ºC and 550ºC and for a duration less than or equal to one second. 10. A process according to claim 7 or 9, wherein the heat treatment is carried out under a protective atmosphere, as is the subsequent cooling. 11. A method according to any one of claims 7 to 10, followed by a pickling step. 12. A method according to any one of claims 7 to 11, which uses means such that when monitoring the ability of such a rope to adhere to a so-called "test rubber compound" before and after said manufacture, the observed deterioration is less than or equal to 60%. 13. Process according to claim 12, in that said so-called test mixture comprises a base material or a base material comprising 100% NR (natural rubber), with an addition of reinforcing additives to achieve a Shore A hardness of more than 70, consisting of a level of sulphur of between 5% and 8% and a level of cobalt of between 0.3% and 0.4% (the percentages are given with reference to the total weight of the elastomer). 14. A method according to any one of claims 7 to 11, which uses such means so that when monitoring the ability of such a rope to adhere to a so-called "test rubber compound" before and after said manufacture, the observed deterioration is less than or equal to a threshold value, said test compound being more sensitive to changes in the covering of the rope than said connecting rubber compound, and said test compound being formulated such that said threshold value should be greater than the deterioration observed in said connecting rubber. 15. "High elongation" steel cord suitable for reinforcing tyres, consisting of hard-drawn carbon steel wires covered with an adhesive coating which promotes adhesion between the steel and the rubber, the diameter of the wires being between 0.05 and 0.8 mm and the carbon content of the steel being between 0.4% and 1%, characterised in that it satisfies the following classification before and after vulcanisation in the tyre: Af = Ae + Ap > 4%, where Af is its so-called "functional" strain, the sum of its elastic strain Ae and its plastic strain Ap. 16. Rope according to claim 15, characterized in that it satisfies the assignment: Rm > 2000 MPa, where Rm is its maximum tensile strength. 17. Rope according to claims 15 or 16, wherein the carbon content of the steel is between 0.7% and 0.9%. 18. A rope according to any one of claims 15 to 17, wherein the casing is made of brass. 19. A rope according to any one of claims 15 to 18, wherein its wires have a strain hardening value ε of less than 3.5. 20. A rope according to any one of claims 15 to 19, consisting of a rope with layers of structure (2+7) or (3+8) consisting of a first layer of two or three wires twisted together, surrounded by a second layer of 7 or 8 wires wound together in a spiral around this first layer. 21. Rope according to any one of claims 15 to 20, which satisfies the assignment Rm > 2200 MPa. 22. Tyres with sidewalls terminating in beads designed to be mounted on a rim, said tyre having a carcass reinforcement (1) passing through the sidewalls and reaching the beads, at least one of the said beads having the following characteristics: - reinforcing carcass elements extending from the radially lower part of the bead to the sidewall, and - at least two stacks of peripheral reinforcing elements (2) laterally adjacent to the reinforcing carcass elements, characterized in that the radially lowest part of the stack, which is axially furthest outward (5), is offset radially upwards relative to the radially lowest part of the adjacent stack (4), and in that said circumferential ropes (2) have a degree of functional elongation Af of more than 4%. 23. A tire according to claim 22, in which the radially lowermost part of each stack is upwardly offset with respect to the radially lowermost part of the axially inwardly adjacent stack. 24. Tyres with sidewalls terminating in beads designed to be mounted on a rim, said tyre comprising a carcass reinforcement (1) extending through the sidewalls and reaching the beads, at least one of said beads has the following characteristics: - reinforcing carcass elements running from the radially lower part of the bead to the sidewall, - at least two stacks of peripheral reinforcing elements (2) laterally adjacent to the reinforcing carcass elements, and - such materials that e1j is the thickness of each product radially separating the radially lowest turn of any stack from the radially lower face of the bead, and e2i is the thickness of each product radially separating the radially lowest turn of any stack from the axially inner face of the bead, and that G1j and G2i are the respective elastic moduli of the products considered, said materials complying with the following rule: e2i * G2i ≤ e1j * G1j characterized in that said circumferential ropes (2) have a degree of functional elongation Af of more than 4%.